Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. Wafers are commonly constructed in layers, where a portion of a circuit is created on a layer and conductive vias are created to electrically connect the circuit to other layers. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit layer. Each layer of the circuit can create or add unevenness to the wafer that is typically smoothed before generating the next circuit layer.
Chemical mechanical planarization (CMP) techniques are used to planarize the raw wafer and each layer of material added thereafter. Available CMP systems are commonly called wafer polishers. Often such a wafer polisher will include a rotating wafer carrier head. The wafer carrier head may bring the wafer into contact with a polishing pad. In a rotary CMP system, the polishing pad may be circularly rotated in the plane of the wafer surface to be planarized. A polishing fluid, such as a chemical polishing agent or slurry containing micro abrasives may be applied to the polishing surface to polish the wafer. The wafer is pressed against the rotating polishing pad and is rotated to polish and planarize the wafer. Another CMP technique uses a linear polisher. Instead of a rotating pad, a moving belt is used to linearly move the pad across the rotating wafer surface.
As the wafer is polished, the polishing pad also becomes smoother or polished. The consistency in polishing multiple wafers is an important aspect of planarization of wafers. To maintain the surface of the polishing pad at a consistent level of abrasiveness, a pad conditioner may be used. The pad conditioner may similarly be pressed into the moving polishing pad. The surface of the pad conditioner may include an abrasive substance, such as diamond grit, to scratch or roughen the surface of the polishing pad. The surface of the polishing pad may be roughened to keep the polishing pad surface under substantially constant conditions, and therefore maintain similar removal rates with different semiconductors.
During the polishing process, slurry is typically dripped or otherwise discharged onto the moving polishing pad using a slurry distribution system. The slurry distribution system is typically upstream of where the wafer polishing occurs. The polishing pad may be porous, or non-porous, and may include grooves or other shapes formed in the polishing pad to assist in getting the dispersed slurry between the wafer and the moving polishing pad. In the case of a porous polishing pad, some of the slurry may soak into the polishing pad.
Since the polishing pad is moving, some of the slurry discharged onto the polishing pad does not adhere to the polishing pad and is thrown off. In addition, because the slurry is on the surface of the polishing pad and the wafer is pressed into the polishing pad, the slurry may not go between the wafer and the polishing pad as is desirable. Instead, the slurry may be pushed aside by the wafer similar to water being pushed aside by the bow of a boat. As a result, additional slurry may need to be discharged onto the polishing pad and additional slurry containment and capture mechanisms may be needed to capture the slurry that is discharged but not used in the polishing process. Slurry is expensive, and less than maximal use of the slurry may unnecessarily raise planarization processing costs.
The consistency of the rate of removal of material from the wafer may be adversely affected due to poor slurry distribution. For example, poor slurry distribution over the surface of the polishing pad may result in delamination (particularly copper with low k films), concentric removal rate profile variations across the wafer, etc. Accordingly, there is a need for systems and methods of maximizing and constantly maintaining the amount of slurry between the polishing pad and the wafer while minimizing the amount of slurry that is discharged, but is unused by the polishing process.